Titanium oxide is widely known as a photocatalyst, but has substantially no photocatalytic function when used in a place where no ultraviolet ray is present. Therefore, there have been made extensive researches concerning tungsten oxide.
As an attempt for allowing tungsten oxide to exhibit a visible light activity or improving the visible light activity, there has been proposed a catalyst obtained by supporting a co-catalyst on a surface of the tungsten oxide. For example, the platinum-supporting tungsten oxide catalyst is capable of exhibiting a photocatalytic activity under irradiation with a visible light (for example, refer to Patent Document 1). Also, the palladium-supporting tungsten oxide catalyst is capable of exhibiting a high catalytic activity to decomposition reaction of toluene (for example, refer to Patent Document 2). However, it is desirable that these noble metals are not used in view of rarity and expensiveness thereof. On the other hand, the tungsten oxide on which copper as a relatively inexpensive metal is supported in the form of a copper ion or copper oxide is capable of exhibiting a photocatalytic activity under irradiation with a visible light (for example, refer to Non-Patent Document 1 and Patent Document 3).
However, when the photocatalyst on which the co-catalyst is supported is used under irradiation with light, metal particles of the co-catalyst tend to be aggregated together, thereby causing discoloration of the photocatalyst as well as problems upon use such as deteriorated retention of properties, poor appearance or the like. As a result, any measures against these drawbacks must be taken.
On the other hand, for the purpose of enhancing an activity of the photocatalyst, the surface of the catalyst has been often subjected to etching treatment. For example, Patent Document 4 discloses the method in which the surface of the catalyst is subjected to photo-etching treatment by irradiation with an ultraviolet light to enhance a photocatalytic activity thereof. However, in this method, use of additional devices such as an ultraviolet generator is needed, and it is therefore difficult to synthesize a large amount of the catalyst, resulting in undesirable process.
Further, Patent Document 5 discloses the process for producing a photocatalyst in which fine particles of gold and/or a transition metal and/or a rare earth element are supported on a metal oxide, and the resulting material is treated with ammonia and then calcined. However, the photocatalyst obtained by the above process tends to suffer from discoloration and fails to exhibit a sufficient activity.